Enhanced distributed channel access parameter variation within restricted access window

ABSTRACT

A wireless access point includes a medium access control circuit that generates a first traffic indication map announcing a restricted access window (RAW). A physical layer device transmits a first beacon, including the first traffic indication map, over a wireless medium. The RAW begins subsequent to the first beacon and ends prior to the next beacon. During the RAW, the physical layer device accesses the wireless medium using a first set of channel access parameters. Outside of the RAW, the physical layer device accesses the wireless medium using a second set of channel access parameters different than the first set. The second set includes channel access values respectively corresponding to access categories. A first frame, associated with a first access category, is transmitted after waiting for a period of time that is based on a channel access value corresponding to the first access category.

CROSS-REFERENCE TO RELATED APPLICATIONS

This present disclosure is a continuation of U.S. patent applicationSer. No. 13/680,831 (now U.S. Pat. No. 9,155,027), filed on Nov. 19,2012, which claims the benefit of U.S. Provisional Application No.61/563,374, filed on Nov. 23, 2011, and U.S. Provisional Application No.61/638,390, filed on Apr. 25, 2012. The entire disclosures of theapplications referenced above are incorporated herein by reference.

FIELD

The present disclosure relates to wireless medium channel access, andmore particularly to extensions to enhanced distributed channel accessof IEEE 802.11.

BACKGROUND

In wireless local area networks, network devices communicate with eachother over a wireless medium that is shared among the network devices.In such cases, in which a wireless medium is shared among multiplenetwork devices, in order to avoid interference among respectivetransmissions of the network devices on the wireless medium, generallyonly one networking device should be permitted to transmit on thewireless medium at any given time. When transmissions interfere, suchoccurrences are commonly referred to as “collisions”. According to IEEEStandard 802.11-2012, the entire disclosure of which is hereincorporated by reference, a carrier sense multiple access withcollision avoidance (CSMA/CA) scheme is therefore defined.

According to CSMA, prior to transmitting, a network device senseswhether the wireless medium is busy—i.e., whether a carrier signal fromanother station's transmission is present on the wireless medium. If thewireless medium appears to be free based on carrier sensing, a collisionavoidance scheme causes the networking device to select a randomizedperiod of time to wait before transmitting. If the wireless mediumremains free during this period of time, the network device beginstransmitting upon conclusion of the period of time. The networkingdevice expects to receive an acknowledgement from the destination of thetransmission. If no acknowledgment is received, the network device mayassume that a collision occurred and therefore attempt to retrytransmission later, possibly after waiting for an even longer randomizedperiod of time.

In FIG. 1, an example illustration of a collision avoidance schemeconsistent with IEEE 802.11 is shown. At time 102, the medium ceases tobe busy. A network device that desires to transmit will watch that themedium remains free until time 106, after a delay called the distributedcoordination function (DCF) interframe space (DIFS). The network devicechooses a random integer between zero and an upper limit, inclusive.This randomized integer may be referred to as a backoff number, abackoff delay, or a backoff interval. The network device then waits forthe selected backoff interval and, assuming the medium remains free,begins transmitting.

In FIG. 1, if the randomly selected backoff interval is zero, thenetwork device can begin transmitting at 106. If the backoff intervalselected is one, the network device can begin transmitting at 110 aftera time period referred to as a slot time. Similarly, if the backoffinterval selected is two, the network device can begin transmitting at114, two slot times after the end of DIFS at 106.

As FIG. 1 demonstrates, DIFS is made up of three time periods, two slottimes and one short interframe space (SIFS). Because network deviceswill normally wait until at least 106 to begin transmitting, even if theselected backoff is zero, an access point can gain precedence overregular network devices by beginning transmission earlier than 106, suchas at 118. The delay from time 102 until time 118 is called a pointcoordination function (PCF) interframe space (PIFS). As seen in FIG. 1,PIFS is formed from SIFS and one slot time.

In FIG. 2, an example transmission following the backoff interval isshown. At 150, a source transmits a request to send (RTS) to adestination. Following a SIFS, during which the destination processesthe RTS and gets ready for transmission, the destination transmits aclear to send (CTS) frame at 154. When the source receives a CTS frame,the source knows that the destination successfully received the RTSframe and that a collision did not occur. Because the RTS frame isrelatively short, the determination of whether a collision has occurredwill be much quicker than if a long data frame experienced a collision.In addition, the CTS frame signals to other network devices, which maynot have received the RTS from the source, that a transmission isexpected and that the medium is busy.

At 158, after a further SIFS, the source begins transmission of data tothe destination. At 162, following another SIFS, the destinationtransmits an acknowledgment to the source if the destination correctlyreceived the data. At 166, the medium is once again free, and othernetwork devices must wait for at least a period of DIFS until 170 beforethey can begin transmission. Between 150 and 170, any network devicesthat had a non-zero backoff would have deferred decrementing theirbackoff until 170. In other words, if another device had a remainingbackoff of two at 150, that backoff would begin decrementing once againat 170. If the medium remains free, after an additional two slot times,the backoff will have decreased to zero and that network device cantransmit.

In order to provide quality of service (QoS)—i.e., giving differentpriorities to different types of traffic—the collision avoidance schemeabove, which is called the distributed coordination function (DCF), hasbeen extended with a scheme called enhanced distributed channel access(EDCA). Under EDCA, two primary changes are made. The first is that DIFSis effectively lengthened for lower priority traffic. In addition, theaverage backoff interval for lower priority traffic is lengthened. Thefollowing table includes the four access categories defined for 802.11QoS. The lowest priority is called background, while the highestpriority is called voice.

Access AC CWmin CWmax Category Description CWmin e.g. CWmax e.g. AIFSNAC_BK Background aCWmin 15 aCWmax 1023 7 AC_BE Best Effort aCWmin 15aCWmax 1023 3 AC_VI Video (aCWmin + 1)/2-1 7 aCWmin 15 2 AC_VO Voice(aCWmin + 1)/4-1 3 (aCWmin + 1)/2-1 7 2

When selecting a backoff interval, the network device selects a randomnumber between 0 and an upper limit. The upper limit is called acontention window (CW) and is initialized to a value called CWmin. Whena transmission fails, likely as a result of collision, the contentionwindow is increased, up to a limit of CWmax. The above tabledemonstrates that the values of CWmin and CWmax are different fordifferent access categories.

The CWmin and CWmax values are defined in terms of parameters aCWmin andaCWmax. Using example values of 15 for aCWmin and 1023 for aCWmax,examples of CWmin and CWmax for each access category are shown. Forexample, for video (AC_VI), the contention window begins at 7 where thebackoff will be randomly selected from the range of 0 to 7, and afterone or more retries, the backoff will be randomly chosen from the rangeof 0 to 15.

The above table also has a column for arbitration interframe spacenumber (AIFSN), which specifies the minimum time that the medium must befree before the backoff interval begins. The arbitration interframespace (AIFS) is specified by the sum of SIFS and AISFN times the slottime. As seen in FIG. 3, the AIFS for an AIFSN of two is the same asDIFS from FIG. 1. The AIFS for an AIFSN of three is one slot time longerthan DIFS. Similarly, the AIFS for an AIFSN of 7 is five slot timeslonger than DIFS. In other words, for best effort (AC_BE) frames, wherethe AIFSN is three, the earliest time a network device could transmitbest effort data is at 204. If the selected backoff is one, best efforttraffic could be transmitted starting at 208. Similarly, if the backoffis two, best effort traffic could begin transmitting at 212, etc.

Using background data (AC_BK) as an example, the contention window,using the example aCWmin and aCWmax values, increases from 15 to amaximum of 1023 as retransmissions are attempted. Once the contentionwindow reaches CWmax, the contention window remains the same for anysubsequent retries. Eventually, after a certain number of retries,transmission of that frame may be abandoned. The function that specifieshow the contention window increases from CWmin to CWmax may be dependenton access category. One example function, which is nearly equivalent todoubling, is (CW+1)*2−1. In other words, the contention window begins at15 and, after an unsuccessful transmission, the CW increases to 31.After another failed transmission attempt, the contention windowincreases to 63, etc.

Referring again to FIG. 3, it can be observed that the total delay fromthe medium becoming free until a transmission begins can be expressed interms of SIFS plus a number of slot times. SIFS is based on the nominaltime that the medium access control (MAC) and physical layer interface(PHY) require in order to receive the last symbol of a frame at the airinterface, process the frame, and respond with the first symbol on theair interface of the earliest possible response frame. IEEE 802.11-2012,section 6.5.4.2.

The slot time is based on four values: aMACProcessingDelay,aAirPropagationTime, aRxTxTurnaroundTime, and aCCATime. TheaMACProcessingDelay value is the maximum time available for the MAC torequest a transmission from the PHY after learning of the end of areception by the PHY. The aAirPropagationTime is twice the propagationtime required for a signal to cross the distance between the mostdistant allowable network devices. The aRxTxTurnaroundTime is themaximum time required by the PHY to change from receiving totransmitting. The aCCATime is the maximum time that a clear channelassessment (CCA) mechanism has available to determine whether the mediumis busy or idle. See IEEE 802.11-2012, section 6.5.4.2. For purposes ofillustration only, examples of these values in the 5 GHz band are 9 μsfor slot time, 16 μs for SIFS, and 4 μs for aCCATime.

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

SUMMARY

A wireless access point includes a medium access control (MAC) circuitand a physical layer device. The MAC circuit is configured to generate afirst traffic indication map. The first traffic indication map announcesa restricted access window. The physical layer device is configured totransmit a first beacon over a wireless medium. The first beaconincludes the first traffic indication map. The restricted access windowbegins subsequent to transmission of the first beacon. The restrictedaccess window ends prior to transmission of a second beacon. The secondbeacon is transmitted subsequent to the first beacon with no interveningbeacons.

The physical layer device is configured to, during the restricted accesswindow, access the wireless medium using a first set of channel accessparameters. The first set of channel access parameters includes a firstshort interframe space and a first slot time. The physical layer deviceis configured to, subsequent to the restricted access window but priorto the transmission of the second beacon, access the wireless mediumusing a second set of channel access parameters. The second set ofchannel access parameters includes a second short interframe space and asecond slot time. The first short interframe space and the second shortinterframe space are equal. The second slot time and the first slot timeare related by an integer multiple, and the integer multiple is greaterthan one.

A method of operating a wireless access point includes generating afirst traffic indication map. The first traffic indication map announcesa restricted access window. The method includes transmitting a firstbeacon over a wireless medium. The first beacon includes the firsttraffic indication map. The restricted access window begins subsequentto transmission of the first beacon. The method includes transmitting asecond beacon subsequent to the first beacon with no interveningbeacons. The restricted access window ends prior to transmission of thesecond beacon.

The method includes, during the restricted access window, accessing thewireless medium using a first set of channel access parameters. Thefirst set of channel access parameters includes a first short interframespace and a first slot time. The method includes, subsequent to therestricted access window but prior to the transmission of the secondbeacon, accessing the wireless medium using a second set of channelaccess parameters. The second set of channel access parameters includesa second short interframe space and a second slot time. The first shortinterframe space and the second short interframe space are equal. Thesecond slot time and the first slot time are related by an integermultiple and the integer multiple is greater than one.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description, the claims and the drawings. Thedetailed description and specific examples are intended for purposes ofillustration only and are not intended to limit the scope of thedisclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graphical illustration of channel access timing parametersaccording to the prior art.

FIG. 2 is a graphical illustration of a timeline of an exampletransmission according to the prior art.

FIG. 3 is a graphical illustration of channel access timing parametersrelating to quality of service according to the prior art.

FIGS. 4A and 4B are graphical illustrations of physical layer preamblesfor 1 MHz and 2 MHz transmissions.

FIGS. 5A-5C are graphical illustrations of channel access timing formixed 1 MHz and 2 MHz transmissions.

FIG. 5D is a graphical illustration of channel access timing for 2 MHztransmissions.

FIGS. 6A-6D are timing diagrams of restricted access windows used forspecifying channel access parameters.

FIGS. 7A-7C are functional block diagrams of a network device.

FIG. 8 is a graphical illustration of relative network device proximity.

FIGS. 9 and 10 are flowcharts of example access point operation.

FIGS. 11 and 12 are flowcharts of example station operation.

FIG. 13 is a flowchart of example network device operation for frametransmission.

DESCRIPTION

For different physical layer interfaces (PHYs), various PHY timingparameters may be different. For example, IEEE Standard 802.11ah(currently in the process of being drafted) may specify a 1 MHz channelPHY and a 2 MHz channel PHY. The clear channel assessment (CCA) time,known as aCCATime, may be different for these two PHYs. This will resultin the short interframe space (SIFS) intervals, as well as the slottimes, being different for the PHYs.

For purposes of illustration only, the preamble of a 1 MHz PHY (see FIG.4A) may have a 4-symbol short training field (STF) 304, where eachsymbol is 40 μs long, while the preamble of a 2 MHz PHY (see FIG. 4B)may have a 2-symbol short training field 312. The symbols in the 2 MHzPHY preamble may also be 40 μs in length. The aCCATime may be based onthe time required to receive half of a short training field and maytherefore be approximately 40 μs for the 2 MHz PHY and 80 μs for the 1MHz PHY. Because of the longer aCCATime, the 1 MHz PHY will have alonger SIFS as well as longer slot times. When the 1 MHz and 2 MHz PHYsare sharing the same wireless channel, 2 MHz transmissions are morelikely to occur than 1 MHz transmissions because the channel accesstimes are on average shorter for the 2 MHz transmissions. This may leadto higher priority data transmitted using the 1 MHz PHY receiving aworse quality of service (QoS) than lower priority data sent using the 2MHz PHY.

One approach is to use the longer 1 MHz channel access times (SIFS andslot times) for both 1 MHz and 2 MHz transmissions. However, this causesall 2 MHz transmissions to be delayed, even if there are no 1 MHztransmissions pending. Therefore, in situations where 1 MHz traffic isnot present, an access point (AP) may allow network devices (also knownas stations or STAs) to make 2 MHz transmissions using shorter channelaccess times specific to the 2 MHz PHY.

When 1 MHz transmissions are present, comparable quality of service forthe same access category (i.e., QoS priority) between 1 MHz and 2 MHzPHYs can be achieved by using channel access parameters based on the 1MHz PHY requirements. The channel access parameters include thearbitration interframe space (AIFS) for each access category andcontention window parameters for each access category. The channelaccess parameters can be made the same by using the same SIFS and slottimes for both 1 MHz and 2 MHz transmissions.

Alternatively, the slot times for 2 MHz transmissions can be madeshorter than the slot times for the 1 MHz transmissions, and the AIFSand contention windows can be equalized by increasing the multipliers(AISFN, aCWmin, and aCWmax) for 2 MHz transmission with respect to 1 MHztransmissions. In other words, if the 2 MHz slot time is half as long asthe 1 MHz slot time, the AIFSNs for 2 MHz transmissions may be doubledwith respect to 1 MHz transmissions. Because the AIFS is the product ofAIFSN and slot time, the halving and doubling cancel out, resulting inthe same AIFS for each access category between 1 MHz and 2 MHztransmissions.

In situations where 2 MHz channels may be plentiful, such as the UnitedStates, 2 MHz transmissions may be more common than 1 MHz transmissions.Further, 1 MHz transmissions are generally used for longer rangetransmission, and may therefore be less likely to interfere with 2 MHztransmissions. In such circumstances, 2 MHz transmissions may beconducted using the shorter channel access parameters specific to 2 MHztransmissions.

Another approach to resolving conflicts between 1 MHz and 2 MHz PHYs isto time-multiplex the wireless channel. For example only, a restrictedaccess window can be used, as described in detail in commonly assignedapplication Ser. No. 13/680,876, filed herewith, and titled “802.11Restricted Access Windows”, the entire disclosure of which is hereincorporated by reference.

The AP may specify a restricted access window following a beacon, where1 MHz channel access parameters are used during the restricted accesswindow period. Outside of the restricted access window, 2 MHz channelaccess parameters are used. This may mean that 1 MHz transmissions areconstrained to only occur within the restricted access window.Alternatively, the restricted access window could be specified to use 2MHz channel access parameters, meaning that 1 MHz transmissions wouldnot occur during the restricted access window. Outside of the restrictedaccess window, 1 MHz times would then be used. 2 MHz transmissions maybe allowed as long as the 2 MHz transmissions were performed using 1 MHzchannel access parameters.

In FIG. 4A, an example 1 MHz PHY preamble is shown. The preambleincludes a short training field (STF) 304 with four symbols and a firstlong training field (LTF) 308, also with four symbols. For example only,each symbol may be 40 μs in length.

In FIG. 4B, an example 2 MHz PHY preamble is shown. The preambleincludes a short training field (STF) 312 with two symbols and a firstlong training field (LTF) 316, also with two symbols. For example only,each symbol may be 40 μs in length.

Because a clear channel assessment (CCA) may require receiving half of ashort training field, the aCCATime for the 1 MHz PHY preamble may beapproximately two symbols multiplied by 40 μs, or 80 μs. Meanwhile, theaCCATime for the 2 MHz PHY preamble may be one symbol multiplied by 40μs, or 40 μs.

For purposes of illustration only, assume that the minimum slot time for2 MHz transmissions, based on the 2 MHz aCCATime of 40 μs, is 90 μs.Also assume that the minimum slot time for 1 MHz transmissions, based onthe 80 μs aCCATime, is 130 μs. In order to synchronize 1 MHz and 2 MHzslot times, the slot time for 1 MHz transmissions can be made an integermultiple of the slot time for 2 MHz transmissions. Note that theaCCATime is the minimum time needed to make a clear channel assessment.Therefore, the slot time can be increased beyond the minimum required byaCCATime. The 1 MHz slot time may therefore be increased to 180 μs,which is double the 2 MHz slot time of 90 μs.

In FIG. 5A, an example is shown where the 1 MHz slot time is twice thatof the 2 MHz slot time. Note that AIFS for an AIFSN of two can beexpressed as SIFS plus two 1 MHz slot times or SIFS plus four 2 MHz slottimes. Similarly, AIFS for an AIFSN of three can be expressed as SIFSplus three 1 MHz slot times or SIFS plus six 2 MHz slot times. After theAIFS for the appropriate access category, random backoff for 2 MHztransmissions can be counted using 2 MHz slot times. Because the 2 MHzslot times are half the size of 1 MHz slot times, the contention windowvalues for 2 MHz transmissions should be doubled. In this way, theactual lengths of contention windows will be the same between 1 MHz and2 MHz transmissions for the same access category.

At 2 MHz slot times such as 350, only 2 MHz transmissions can begin. At1 MHz slot times, such as 354, 1 MHz transmissions can begin and,because the slot times are multiple of each other, 2 MHz transmissionscan also begin. Note that when both 1 MHz and 2 MHz transmissions arepresent, there may be situations in which they conflict. For example, ifa 1 MHz transmission is begun at 358, a station backing off until 362before making a 2 MHz transmission may not have observed the 1 MHztransmission beginning at 358 by the time the 2 MHz transmission beginsat 362. This situation will likely result in a collision.

FIG. 5B shows one approach to eliminating this sort of collision:prevent 2 MHz transmissions from beginning at 2 MHz slot times that donot line up with 1 MHz slot times. In other words, if the random backofffor a 2 MHz transmission would cause the 2 MHz transmission to begin at380, an additional 2 MHz slot time may be added to the backoff intervalto delay the 2 MHz transmission until 384. As a result, if a 1 MHztransmission had begun at 388, the transmission at 384 would be deferredbecause there was sufficient time to detect the 1 MHz transmissionbeginning at 388.

In FIG. 5C, another implementation is shown where the 1 MHz and 2 MHzslot times are set to be equal to each other. Assuming that the SIFS isalso set equal between the 1 MHz and 2 MHz transmissions, the equal slottimes will mean that the AIFS values will line up for the same accesscategory across 1 MHz and 2 MHz transmissions.

The SIFS and aSlotTime may be defined in units of symbols or halfsymbols instead of in units of absolute time, such as microseconds ortens of microseconds. For example, with a symbol duration of 40 μs, aSIFS of 80 μs could be defined as 2 symbols or as 4 half-symbols. Thismay require fewer bits than encoding the number 80. Further, theduration of a transmission sequence is based on a series of physicallayer convergence procedure (PLCP) Protocol Data Units (PPDUs) andassociated interframe spaces and can therefore be expressed as aninteger number of symbols. Using units of symbols or half-symbols mayallow for a frame's duration field to use fewer bits.

In various implementations, 1 MHz transmissions may be restricted tohave at most one data exchange at a time. This may be achieved bysetting a transmission opportunity (TXOP) field to zero, for all accesscategories, in 1 MHz transmissions.

In FIG. 5D, an example is shown where channel access parameters specificto 2 MHz transmissions are used. The SIFS may be shorter when only theaCCATime of 2 MHz transmissions needs to be considered. In addition, theAIFS may be set based on the shorter 2 MHz slot times. As a result, theAIFS for a given access category of data is reduced when compared to themixed PHY situations of FIGS. 5A-5C.

In FIG. 6A, a traffic indication map (TIM) or delivery trafficindication map (DTIM) beacon 404 establishes one or more restrictedaccess windows. The DTIM may be followed by a period 408 where if anybroadcast or multicast packets are queued for transmission by the AP,the broadcast and multicast packets are transmitted.

Following the broadcast/multicast window 408, a first restricted accesswindow (RAW) 412 begins. During the first RAW 412, 1 MHz channel accessparameters are used and therefore 1 MHz transmissions are allowed. Thefirst RAW 412 is followed by a second RAW 416, during which 2 MHzchannel access parameters are used. After the second RAW 416 concludes,stations may revert to the default channel access parameters establishedby the AP, which may be the same as those in the first RAW 412, those inthe second RAW 416, or a different set of access parameters.

The first RAW 412 may be restricted to only 1 MHz transmissions. Thismay be because the AP desires to give 1 MHz transmissions a chance tooccur without conflicting with 2 MHz transmissions. Additionally oralternatively, the channel access parameters used in the first RAW 412of FIG. 6A may not be synchronized with 2 MHz channel access parameters.For example only, the slot time used in the first RAW for 1 MHztransmissions may not be an integer multiple of the slot time used inthe second RAW 416 for 2 MHz transmission.

Note that the first RAW 412 begins subsequent to transmission of thebeacon 404. The second RAW 416 begins subsequent to or coincident withthe end of the first RAW 412. Both the first RAW 412 and the second RAW416 end before transmission of a second beacon 420. Note that the secondbeacon 420 is the beacon transmitted directly subsequent to the beacon404, there being no intervening beacons between the beacon 404 and thesecond beacon 420. In various implementations, beacons are nottransmitted during RAWs. Therefore, RAWs may be scheduled to end beforean approximate target beacon transmission time (TBTT) so thattransmission of a beacon is not delayed while waiting for a RAW to end.

In FIG. 6B, a DTIM 450 establishes a first RAW 454 during which 1 MHzchannel access parameters are used. The 1 MHz channel access parametersare synchronized with 2 MHz channel access parameters and thereforeeither 1 MHz or 2 MHz transmissions may occur within the first RAW 454.Returning now to FIG. 6B, a second RAW 458 follows the first RAW and mayuse the same channel access parameters as the second RAW 416 of FIG. 6A.In FIG. 6A, the timing of the first RAW 412 and the second RAW 416 maybe reversed such that 2 MHz transmissions occur first. Similarly, inFIG. 6B, the second RAW 458 may be reversed with the first RAW 454. Thisorder may be determined dynamically by the AP or may be predefined by astandard.

In FIG. 6C, a DTIM 504 establishes a single RAW 508. Within the RAW 508,2 MHz channel access parameters are used. Once the RAW 508 ends, 1 MHzchannel access parameters are used. To allow 2 MHz transmissions outsideof the RAW 508, the 1 MHz channel access parameters are synchronizedwith 2 MHz channel access parameters.

Alternatively, in FIG. 6D, a DTIM 524 specifies a RAW 528 during which 1MHz channel access parameters are used. Outside of the RAW 528, 2 MHzchannel access parameters are used. The RAW 528 may be restricted toonly 1 MHz transmissions, in which case the channel access parametersused within the RAW 528 may be unsynchronized with 2 MHz transmissions.This may allow for shorter slot times than if the 1 MHz slot timesneeded to be an integer multiple of the 2 MHz slot times forsynchronization purposes.

In FIG. 7A, a network device 600 includes a host processor 604 thatinterfaces with memory 608. The host processor 604 sends data to andreceives data from a medium access control (MAC) circuit 612. The MACcircuit 612 communicates with a wireless medium using a first ratephysical layer interface (PHY) 616 and also interacts with the wirelessmedium using a second rate PHY 620. For example only, the first rate PHY616 may operate with 1 MHz channels, while the second rate PHY 620 mayoperate with 2 MHz channels. A 1 MHz channel may span the higherfrequency half of a 2 MHz channel or the lower frequency half of the 2MHz channel.

In FIG. 7B, a network device 640 includes a host processor 604 and amemory 608, which may be configured similarly to the host processor 604and the memory 608 of FIG. 7A. The host processor 604 sends data to andreceives data from a MAC circuit 644. The MAC circuit 644 communicateswith a physical layer medium using a multi-rate PHY 648, which maysupport both 1 MHz and 2 MHz channel communication.

In FIG. 7C, a network device 680 includes a host processor 604 and amemory 608, which may be configured similarly to the host processor 604and the memory 608 of FIGS. 7A and 7B. The host processor 604 sends datato and receives data from a MAC circuit 684. The MAC circuit 684communicates with a wireless medium using a first rate PHY 688. Thefirst rate PHY 688 sends and receives data using a first rate but may beable to detect transmissions made using a second rate. This may allowthe first rate PHY 688 to avoid collisions and may also inform the firstrate PHY 688 and the MAC circuit 684 of what channel access parametersare appropriate to use in the surrounding environment.

In alternative implementations, the first rate PHY 688 may lack anyexplicit second rate detection. The first rate PHY 688 would thereforerely on an expected lack of interference from the second rate. The firstrate PHY 688 may, in various implementations, rely on inherent thoughimperfect detection of channel usage by second rate transmissions whileusing the first rate channel usage detection mechanisms.

In FIG. 8, graphical illustration of relative network device proximityis shown. A first AP 700 forms an infrastructure basic service set (BSS)with a first station 704 and a second station 708. The first AP 700communicates with the first station 704 and the second station 708 usinga 2 MHz channel. Meanwhile, 1 MHz communication, which may be used overlong distances, such as 1 km, is used between a second AP 720 and athird station 724.

In certain situations, the first AP 700 and the first and secondstations 704 and 708 may be able to ignore the 1 MHz transmissionsbetween the second AP 720 and the third station 724 because those 1 MHztransmissions are geographically remote. The first and second APs 700and 720 and the first, second, and third stations 704, 708, and 724 mayeach be generally implemented by one or more of the network devices inFIGS. 7A-7C.

In FIG. 9, a flowchart of example access point operation starts at 804,where the AP monitors for lower rate transmissions, such as 1 MHztransmissions. Control continues at 808 where, if lower ratetransmissions are detected, control transfers to 812; otherwise, controltransfers to 816. At 812, control sets channel access parameters basedon the lower rate and continues at 820 where the channel accessparameters are transmitted to stations. Control continues at 824, wherea timer is reset. The timer keeps track of how long it has been sincelower rate transmissions were observed.

Control continues at 828, where control monitors for lower ratetransmission. Control continues at 832, where if lower ratetransmissions are detected, control transfers to 835; otherwise, controltransfers to 840. At 836, control resets the timer and continues at 828.At 840, if the timer exceeds the threshold, lower speed transmissionshave not been detected for that threshold amount of time and controltransfers to 816; otherwise, control returns to 828 to continuemonitoring for lower rate transmissions.

At 816, control sets channel access parameters based on the higher rateand continues at 850 where the channel access parameters are transmittedto the station. Control continues at 854 and monitors for lower ratetransmissions. At 858, if lower rate transmissions are detected, controltransfers to 812; otherwise, control returns to 854 to continuemonitoring for lower rate transmissions.

In FIG. 10, another flowchart depicts example AP operation usingrestricted access windows. Although one restricted access window isdemonstrated in FIG. 10, more restricted access windows may be definedand used. Control begins at 904, where if the target beacon transmissiontime has been reached, control transfers to 908; otherwise, controlremains at 904. At 908, control determines whether the wireless mediumis free. If so, control continues at 912; otherwise, control remains at908. At 912, control transmits a traffic indication map (TIM), which maybe a delivery traffic indication map (DTIM).

The DTIM may indicate whether broadcast or multicast data is queued.Control continues at 916, where if broadcast or multicast data isqueued, control transfers to 920; otherwise, control continues at 924.At 920, control waits until the medium is free and then transfers to928. At 928, control transmits the queued broadcast or multicast data,and continues at 924.

At 924, control uses first rate channel access parameters whentransmitting frames. Control continues at 932, where control waits untilthe restricted access window has expired, at which point controltransfers to 936. At 936, control uses second rate channel accessparameters for transmitting frames. For example only, the first rate maycorrespond to 1 MHz and the second rate may correspond to 2 MHz, or viceversa. The techniques of FIG. 10 may be used in combination with thosein FIG. 9. For example only, when lower rate transmissions are detectedaccording to FIG. 9, the restricted access window may be enabled in FIG.10. Otherwise, control in FIG. 10 may omit specifying the restrictedaccess window and simply use the 2 MHz channel access parameters for allcommunication after the DTIM.

In FIG. 11, a flowchart depicts example operation of a station. Controlbegins at 1004, where control waits until a TIM or DTIM is received.Control then continues at 1008, where if a restricted access window isspecified, control transfers to 1012; otherwise, control transfers to1016. At 1012, control uses first rate channel access parameters.Control then waits at 1020 until the restricted access window hasexpired.

Control then continues at 1024, where second rate channel accessparameters are used. Control then returns to 1004. At 1016, control useshigher rate channel access parameters. Alternatively, the default whenno specific access window is specified may be lower rate parameters. Thefirst and second rates of FIG. 11 may correspond to 1 MHz and 2 MHzrespectively, or vice versa.

In FIG. 12, another flowchart depicting example operation of a stationis presented. At 1104, control waits for a frame to be received from ahost processor. At 1108, a MAC device determines whether the destinationof the frame is considered to be a remote node. If so, control transfersto 1112, where control attempts to transmit the frame using the lowerrate, such as 1 MHz. Control continues at 1116, where if transmission issuccessful, control returns to 1104. If transmission is not successful,control continues at 1120, where error handling is performed. Errorhandling may include incrementing a retry counter and reattemptingtransmission.

At 1108, if the destination of the frame is not considered a remotenode, control transfers to 1124, where control attempts to transfer theframe using the higher rate, such as 2 MHz. If transmission issuccessful at 1128, control returns to 1104; otherwise, controltransfers to 1132. If the number of retries for transmitting this frameexceeds threshold, control transfers to 1136; otherwise, control returnsto 1124. At 1136, control marks the destination as a remote node andattempts to transmit the frame at the lower rate at 1112.

In FIG. 13, another flowchart depicts example operation of a station.The operation of FIG. 13 may be invoked when transmission of a frame isrequested. Control begins at 1204, where the arbitration interframespace (AIFS) is determined based on the access category of the frame. Inaddition, the contention window is determined based on the accesscategory. In FIG. 13, the frame is to be transmitted using a 2 MHzchannel and the channel access parameters are established as describedin FIG. 5B, where the slot time of the 2 MHz rate is double that of theslot time of the 1 MHz rate. Control continues at 1208, where controldetermines the backoff by selecting a random number between zero and thecontention window, inclusive.

Control continues at 1212, where if the medium is free for a period oftime specified by AIFS, control transfers to 1216; otherwise, controlremains at 1212. At 1216, if the backoff is greater than zero, controltransfers to 1220. At 1220, if the medium remains free for the followingslot, the backoff is decremented by one at 1224. Control then returns to1216. At 1220, if the medium became busy during the slot, decrementingis halted and control returns to 1212. If the backoff was zero at 1216,control transfers to 1228.

At 1228, control determines whether the slot lines up with a lower rateslot. If so, control continues at 1232; otherwise, control transfers to1236. At 1236, the backoff is set to one, thereby delaying thetransmission until the following slot, which will be in alignment with alower rate slot. Control then transfers to 1220 to ensure that themedium remains free for that delay slot.

At 1232, control determines whether a virtual collision is present. Avirtual collision occurs when frames of different access categories areboth scheduled to be transmitted beginning at the same slot. Although nophysical collision takes place and retry counters are not updated, thevirtual collision is otherwise treated as a collision, thereby causingthe contention window to be adjusted at 1240. The contention window isadjusted based on the access category of the frame and control returnsto 1208 to determine a new backoff time based on the adjusted contentionwindow. If no virtual collision is present at 1232, control continues at1244, where the frame is transmitted using the higher rate, such as 2MHz. At 1248, if the transmission is successful, control ends;otherwise, a collision is assumed and control transfers to 1240 toadjust the contention window.

The foregoing description is merely illustrative in nature and is in noway intended to limit the disclosure, its application, or uses. Thebroad teachings of the disclosure can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited since othermodifications will become apparent upon a study of the drawings, thespecification, and the following claims. For purposes of clarity, thesame reference numbers will be used in the drawings to identify similarelements. As used herein, the phrase at least one of A, B, and C shouldbe construed to mean a logical (A or B or C), using a non-exclusivelogical OR. It should be understood that one or more steps within amethod may be executed in different order (or concurrently) withoutaltering the principles of the present disclosure.

In this application, including the definitions below, the term modulemay be replaced with the term circuit. The term module may refer to, bepart of, or include an Application Specific Integrated Circuit (ASIC); adigital, analog, or mixed analog/digital discrete circuit; a digital,analog, or mixed analog/digital integrated circuit; a combinationallogic circuit; a field programmable gate array (FPGA); a processor(shared, dedicated, or group) that executes code; memory (shared,dedicated, or group) that stores code executed by a processor; othersuitable hardware components that provide the described functionality;or a combination of some or all of the above, such as in asystem-on-chip.

The term code, as used above, may include software, firmware, and/ormicrocode, and may refer to programs, routines, functions, classes,and/or objects. The term shared processor encompasses a single processorthat executes some or all code from multiple modules. The term groupprocessor encompasses a processor that, in combination with additionalprocessors, executes some or all code from one or more modules. The termshared memory encompasses a single memory that stores some or all codefrom multiple modules. The term group memory encompasses a memory that,in combination with additional memories, stores some or all code fromone or more modules. The term memory may be a subset of the termcomputer-readable medium. The term computer-readable medium does notencompass transitory electrical and electromagnetic signals propagatingthrough a medium, and may therefore be considered tangible andnon-transitory. Non-limiting examples of a non-transitory tangiblecomputer readable medium include nonvolatile memory, volatile memory,magnetic storage, and optical storage.

The apparatuses and methods described in this application may bepartially or fully implemented by one or more computer programs executedby one or more processors. The computer programs includeprocessor-executable instructions that are stored on at least onenon-transitory tangible computer readable medium. The computer programsmay also include and/or rely on stored data.

What is claimed is:
 1. A wireless access point comprising: a mediumaccess control (MAC) circuit configured to generate a first trafficindication map, wherein the first traffic indication map announces arestricted access window; and a physical layer device configured totransmit a first beacon over a wireless medium, wherein the first beaconincludes the first traffic indication map, wherein the restricted accesswindow begins subsequent to transmission of the first beacon, andwherein the restricted access window ends prior to transmission of asecond beacon, wherein the second beacon is transmitted subsequent tothe first beacon with no intervening beacons, during the restrictedaccess window, access the wireless medium using a first set of channelaccess parameters, and outside of the restricted access window, accessthe wireless medium using a second set of channel access parameters,wherein the second set of channel access parameters is different thanthe first set of channel access parameters, the second set of channelaccess parameters includes a plurality of channel access valuesrespectively corresponding to a plurality of access categories, a firstframe is associated with a first access category of the plurality ofaccess categories, and the first frame is transmitted after waiting fora period of time that is based on a channel access value of theplurality of channel access values corresponding to the first accesscategory.
 2. The wireless access point of claim 1, wherein, for stationsin a basic service set corresponding to the wireless access point, thefirst beacon instructs the stations to: during the restricted accesswindow, access the wireless medium using the first set of channel accessparameters; and outside of the restricted access window, access thewireless medium using the second set of channel access parameters. 3.The wireless access point of claim 1, wherein: the plurality of channelaccess values is a plurality of arbitration interframe space values; theplurality of arbitration interframe space values specify, for eachaccess category, a minimum time that the wireless medium must be freebefore a random backoff interval begins; and frame transmission isdelayed until the random backoff interval ends.
 4. The wireless accesspoint of claim 3, wherein: the second set of channel access parametersincludes a second plurality of channel access values respectivelycorresponding to the plurality of access categories; the secondplurality of channel access values is a plurality of contention windowvalues; and the plurality of contention window values determine, foreach access category, a length of the random backoff interval.
 5. Thewireless access point of claim 4, wherein: the second set of channelaccess parameters includes a third plurality of channel access valuesrespectively corresponding to the plurality of access categories; thesecond plurality of channel access values is a plurality of minimumcontention window values; the plurality of minimum contention windowvalues define, for each access category, an initial upper boundary ofthe random backoff interval; the third plurality of channel accessvalues is a plurality of maximum contention window values; and theplurality of maximum contention window values define, for each accesscategory, a final upper boundary of the random backoff interval.
 6. Thewireless access point of claim 1, wherein the plurality of channelaccess values is one of: a plurality of minimum contention window valuesrespectively corresponding to the plurality of access categories; and aplurality of maximum contention window values respectively correspondingto the plurality of access categories, wherein an upper boundary of arandom backoff interval is bounded by at least one of (i) one of theplurality of minimum contention window values and (ii) one of theplurality of maximum contention window values, and wherein frametransmission is delayed until the random backoff interval ends.
 7. Thewireless access point of claim 1, wherein: the first traffic indicationmap also announces a second restricted access window, wherein the secondrestricted access window ends prior to transmission of the secondbeacon; and the physical layer device is configured to, outside of therestricted access window and the second restricted access window, accessthe wireless medium using the second set of channel access parameters.8. The wireless access point of claim 7, wherein: the physical layerdevice is configured to, during the second restricted access window,access the wireless medium using a third set of channel accessparameters; and the third set of channel access parameters is differentthan the second set of channel access parameters.
 9. The wireless accesspoint of claim 7, wherein the second restricted access window beginscoincident with an end of the restricted access window.
 10. The wirelessaccess point of claim 7, wherein: a first set of stations forms a basicservice set with the wireless access point; the first beacon instructs afirst subset of the first set of stations not to transmit during therestricted access window; and the first beacon instructs a second subsetof the first set of stations not to transmit during the secondrestricted access window.
 11. The wireless access point of claim 1,wherein: a first set of stations forms a basic service set with thewireless access point; and the first beacon instructs a first subset ofthe first set of stations not to transmit during the restricted accesswindow.
 12. A method of operating a wireless access point, the methodcomprising: generating a first traffic indication map, wherein the firsttraffic indication map announces a restricted access window;transmitting a first beacon over a wireless medium, wherein the firstbeacon includes the first traffic indication map, wherein the restrictedaccess window begins subsequent to transmission of the first beacon, andwherein the restricted access window ends prior to transmission of asecond beacon, wherein the second beacon is transmitted subsequent tothe first beacon with no intervening beacons; during the restrictedaccess window, accessing the wireless medium using a first set ofchannel access parameters; and outside of the restricted access window,accessing the wireless medium using a second set of channel accessparameters, wherein the second set of channel access parameters isdifferent than the first set of channel access parameters, the secondset of channel access parameters includes a plurality of channel accessvalues respectively corresponding to a plurality of access categories, afirst frame is associated with a first access category of the pluralityof access categories, and the first frame is transmitted after waitingfor a period of time that is based on a channel access value of theplurality of channel access values corresponding to the first accesscategory.
 13. The method of claim 12, wherein, for stations in a basicservice set corresponding to the wireless access point, the first beaconinstructs the stations to: during the restricted access window, accessthe wireless medium using the first set of channel access parameters;and outside of the restricted access window, access the wireless mediumusing the second set of channel access parameters.
 14. The method ofclaim 12, wherein: the plurality of channel access values is a pluralityof arbitration interframe space values; the plurality of arbitrationinterframe space values specify, for each access category, a minimumtime that the wireless medium must be free before a random backoffinterval begins; and frame transmission is delayed until the randombackoff interval ends.
 15. The method of claim 14, wherein: the secondset of channel access parameters includes a second plurality of channelaccess values respectively corresponding to the plurality of accesscategories; the second plurality of channel access values is a pluralityof contention window values; and the plurality of contention windowvalues determine, for each access category, a length of the randombackoff interval.
 16. The method of claim 15, wherein: the second set ofchannel access parameters includes a third plurality of channel accessvalues respectively corresponding to the plurality of access categories;the second plurality of channel access values is a plurality of minimumcontention window values; the plurality of minimum contention windowvalues define, for each access category, an initial upper boundary ofthe random backoff interval; the third plurality of channel accessvalues is a plurality of maximum contention window values; and theplurality of maximum contention window values define, for each accesscategory, a final upper boundary of the random backoff interval.
 17. Themethod of claim 12, wherein the plurality of channel access values isone of: a plurality of minimum contention window values respectivelycorresponding to the plurality of access categories; and a plurality ofmaximum contention window values respectively corresponding to theplurality of access categories, wherein an upper boundary of a randombackoff interval is bounded by at least one of (i) one of the pluralityof minimum contention window values and (ii) one of the plurality ofmaximum contention window values, and wherein frame transmission isdelayed until the random backoff interval ends.
 18. The method of claim12, wherein: the first traffic indication map also announces a secondrestricted access window, wherein the second restricted access windowends prior to transmission of the second beacon; and the methodincludes, outside of the restricted access window and the secondrestricted access window, accessing the wireless medium using the secondset of channel access parameters.
 19. The method of claim 18, furthercomprising, during the second restricted access window, accessing thewireless medium using a third set of channel access parameters, whereinthe third set of channel access parameters is different than the secondset of channel access parameters.
 20. The method of claim 18, whereinthe second restricted access window begins coincident with an end of therestricted access window.
 21. The method of claim 18, wherein: a firstset of stations forms a basic service set with the wireless accesspoint; the first beacon instructs a first subset of the first set ofstations not to transmit during the restricted access window; and thefirst beacon instructs a second subset of the first set of stations notto transmit during the second restricted access window.
 22. The methodof claim 12, wherein: a first set of stations forms a basic service setwith the wireless access point; and the first beacon instructs a firstsubset of the first set of stations not to transmit during therestricted access window.
 23. A method of operating a wireless accesspoint, the method comprising: transmitting a first beacon over awireless medium, wherein the first beacon announces a time period,wherein the time period begins subsequent to transmission of the firstbeacon, and wherein the time period ends prior to transmission of asecond beacon, wherein the second beacon is transmitted subsequent tothe first beacon with no intervening beacons; during the time period,accessing the wireless medium using a first set of channel accessparameters; and outside of the time period, accessing the wirelessmedium using a second set of channel access parameters, wherein thesecond set of channel access parameters is different than the first setof channel access parameters, the second set of channel accessparameters includes a plurality of channel access values respectivelycorresponding to a plurality of access categories, a first frame isassociated with a first access category of the plurality of accesscategories, and the first frame is transmitted after waiting for aperiod of time that is based on a channel access value of the pluralityof channel access values corresponding to the first access category. 24.The method of claim 23, wherein, for stations in a basic service setcorresponding to the wireless access point, the first beacon instructsthe stations to: during the time period, access the wireless mediumusing the first set of channel access parameters; and outside of thetime period, access the wireless medium using the second set of channelaccess parameters.
 25. The method of claim 23, wherein: the plurality ofchannel access values is a plurality of arbitration interframe spacevalues; the plurality of arbitration interframe space values specify,for each access category, a minimum time that the wireless medium mustbe free before a random backoff interval begins; and frame transmissionis delayed until the random backoff interval ends.
 26. The method ofclaim 25, wherein: the second set of channel access parameters includesa second plurality of channel access values respectively correspondingto the plurality of access categories; the second plurality of channelaccess values is a plurality of contention window values; and theplurality of contention window values determine, for each accesscategory, a length of the random backoff interval.
 27. The method ofclaim 26, wherein: the second set of channel access parameters includesa third plurality of channel access values respectively corresponding tothe plurality of access categories; the second plurality of channelaccess values is a plurality of minimum contention window values; theplurality of minimum contention window values define, for each accesscategory, an initial upper boundary of the random backoff interval; thethird plurality of channel access values is a plurality of maximumcontention window values; and the plurality of maximum contention windowvalues define, for each access category, a final upper boundary of therandom backoff interval.
 28. The method of claim 23, wherein theplurality of channel access values is one of: a plurality of minimumcontention window values respectively corresponding to the plurality ofaccess categories; and a plurality of maximum contention window valuesrespectively corresponding to the plurality of access categories,wherein an upper boundary of a random backoff interval is bounded by atleast one of (i) one of the plurality of minimum contention windowvalues and (ii) one of the plurality of maximum contention windowvalues, and wherein frame transmission is delayed until the randombackoff interval ends.
 29. The method of claim 23, wherein: the firstbeacon also announces a second time period, wherein the second timeperiod ends prior to transmission of the second beacon; and the methodincludes, outside of the time period and the second time period,accessing the wireless medium using the second set of channel accessparameters.
 30. The method of claim 29, further comprising, during thesecond time period, accessing the wireless medium using a third set ofchannel access parameters, wherein the third set of channel accessparameters is different than the second set of channel accessparameters.
 31. The method of claim 29, wherein the second time periodbegins coincident with an end of the time period.
 32. The method ofclaim 29, wherein: a first set of stations forms a basic service setwith the wireless access point; the first beacon instructs a firstsubset of the first set of stations not to transmit during the timeperiod; and the first beacon instructs a second subset of the first setof stations not to transmit during the second time period.
 33. Themethod of claim 23, wherein: a first set of stations forms a basicservice set with the wireless access point; and the first beaconinstructs a first subset of the first set of stations not to transmitduring the time period.